Magnetic disk drive sector servo systems are set up with the servo sector tracks offset from the data sector tracks by a half track spacing such that the device read/write head is centered on a data track when so aligned as to receive signals of equal strength from interfacing sector servo track portions. Since the sector servo information is used at the margins of the written tracks it is imperative that the servo data be written to the full track width whereas data tracks use only about 80 percent of the intertrack spacing and are accordingly narrower than the track pitch.
There are two common methods for initializing the disk or disks with sector servo information. The product head or a head narrower than the intertrack pitch may be used by making two recording passes over each sector to be recorded. This requires precise alignment of the corresponding pulses which can never be wholly achieved and is often less than acceptable.
The other method is to use a special write head and actuator, a servo track writer which is especially designed to write the full width of the intertrack pitch in a single pass. This is a precision device that can be accurately positioned and aligned. The limitation with the use of this device is that the product head and actuator assembly is normally not such a precision device and can not be made so without undue expense and effort. The product head assembly does not have heads in perfect alignment along an axis or axes parallel to the axis of rotation of the disk or disks. In addition the linear motion of the assembly while moving from track to track is not likely to be exactly radial or precisely aligned with any other path selected for access. The result is the tracks that should be vertically aligned between surfaces are not, nor are the circumferential positions of the various heads aligned. Both these conditions require resynchronizing when switching between tracks even when supposedly aligned parallel to the axis of rotation.
It is important to have each data, transducer, read/write head properly, simultaneously aligned with the corresponding track or cylinder. In use it is the practice to record information, whenever possible on the same track or on a track that can be accessed by switching heads, rather than on a track that requires an actuator access. This obviously enhances the data rate of either reading or writing since an actuator assembly access movement is not required. Accordingly, studies have shown that in practice, 60% of data accesses are made without moving or accessing the transducer assembly. Accordingly, it is important to have each disk drive head or transducer correctly positioned over a track or cylinder so that electrically switching between heads does not also require an access to correctly position the newly addressed head.